Controlled feedback circuit



1.. muck 2,269,249

CONTROLLED FEEDBACK CIRCUIT Jan. 6, 1942.

Filed Nov.- 29, 19 38 I 7 PRE-DETECTOR NEGATIVE FEED BACK mwszvrogg. THAR BRU CK ATTORNEK Patented Jan. 6, 1942 Lethar firiiclnjBcriin,Germany,;assignor to Tele- Q funken Gesellschaft fur DrahtloseTelegraphic m. i b. 11., Berlin, "Germany; a corporation .6!

Appucaiormma 29, 1938, Serial No. 242,885 1 InGe 'mauy December 16,11937, r

.1" claims. ,(01. 250420) There has been disclosed 'anamplifier in whichth'e'volumeor gain iscontrolledybyrchange of the grid biasin'gipotential in ,one, or several, an p'li fier tubes,-a-ndcomprising-anti+regeneration or negative regeneration' means, and inwhich the fraction of the output potential which=is in negative feedbackrelation to the input end is made variable in such away --that itincreases as the gain diminishes. The advantage; residingin thisarrangement is that an undesiredflreduction of the negative feedback,and thus the impairment of the correction of distortion, with:decreasing gain is avoided. Conditionsmay be made so that grid of; tubeV2. The amplifiedaudio alternat in'g voltage isjfed tot-he loudspeaker-S by way of thetransform'er T'; Wound on the said transthe valueyordegree, of the negative feedback which is' -given by the relationshipbetween the negative feedback potential and the input poten* tial, andthus also the amount of correction or compensation of distortion, stayconstant oreven decreasing with diminishing gain so that the volumecontrol is boosted. In the particular ar rangement disclosed in theipastthe outputmov tentialwas applied to a voltagedividen one arm of whichcontained a resistance having. =anegative resistance-currentcharacteristic.- .,Thi-s 'resistance was inserted in the cathode leadof; the

controlled tube and was regulated by the plate direct current. Y

The identical effect, accordingto theupresent invention, is accomplishedmore simply by impressing the alternating,potential-serving for thenegative ieedback uponthescreengrid of a gain controlled tube by way ofa resistancewhiohis of l at least the same ma'g nitude as the innerresistance of the screen :grid to cathodepathi The operation and effectof the arrangement of the invention is predicated uponthe-fact that theinner resistance is connected .so as to be used a s part of the voltagedivider across which the potential subject to-negative-ieedback isimpressed. By the variation of-Qt'he grid biasing voltage the innerresistance 'is-regulated ins'uch a way'that in the presence of morenegative biasing voltage, that is lower gain, the inner resistancebecomes higher and thus the fraction of the potential in negativefeedback also higher.

former is an additional;secondary winding W. One end of the latter isconnected with --a source of-positivepotential. The other end thereof isassociated by way -of-resistance -R1- with the screen gridof tubefvhThepolarityof the said additional winding must be so-chosen that thealternating potential impressed upon the screen grid' is in phaseopposition to the alternating potential acting uponthe control 1 grid;Resistance -R1 should be :at leastas high as, or preferably higher"than; the" inner resistance of the 7 screen gridto cathode path. Insuch case "the iraction or theoutput-potenti-al which is associated:with the screengrid in negative ,Ieedback is roughly proportional to:the inner resistance. of the screen grid to cathode path, onci-rcuit,and it variesin accordance with the inner-resistance;

in other words, it increases as the biasing potential-ofthe-control-gridbecomes more negative. The constants may be chosen insuch away that the 1 ratio between the negative feedback potential andthe input potentiaLinsidethe regulat- =ing range, will not diminishesthe gain diminishes, but will,- ,on the contrary,- become veven higher.In the presentinstance, this means that automatic volume control isboosted by the change in negativeieedback. Another vfact .is that vwhena powerful transmitter is received the negative regeneration, and thusthe compensation both of linear as well as of non-linear distortion,becomes particularly high. a a

If the direct current voltage 1s impressed upon the screen grid by-wayof a separate resistance An exemplified embodiment of the invention 1 isshown in the drawing. From'the load resistance R which, in series withthe radio frequency choke coil L, is connected in parallel relation tothe signal detector diode D of a radio receiver, both the audiopotential as well as the negative direct current potential are impressedupon the control grid of the pentode tube V1. 1

The anode of this tube is connected by the usual resistance-condensercoupling with the control.

rather than by way of resistanceR- then'such separate resistance shouldbe-chosen high in comparison with the inner resistance of the screengrid to cathode path of tube V1 to the end that, I

on the one hand,.a high over-all outer resistance may be obtained, andthat, on, the other hand,

the-inner resistance may not be short circuited. An expedient plan is toinsert inthe lead from the additional winding W to the screen grid,

frequency-dependent means in order that the negative regenerationefie'ct may be brought about in definite frequency ranges, moreparticularly and predominantly upon the medium range of tonalfrequencies.

From the load resistance R the direct current voltage may be fed by wayof an audio filter network to the radio frequency and intermediatefrequency amplifier tubes of the receiver for the purpose of gainregulation, where automatic vol- 1. In a'signal amplifier of the typecomprising a tube having at least a cathode, signal grid, screen gridand plate, a signal input circuit coupled to the signal grid, a signaloutput circuit coupled to the plate, means for feeding back to thescreen grid from the output circuit a signal potential in degenerativephase with the signal potential applied to the signal grid, saidfeedback means comprising a potential divider including the screen gridto cathode resistance of said tube, and means for simultaneously increasing the negative bias of said signal grid thereby decreasing the tubegain thereby to increase the magnitude of said resistance whereby thevalue of said degenerative potential is increased.

2. In a signal amplifier of the type comprising a tube having at least acathode, signal grid,

screen grid and plate, a signal input circuit coupled to the signalgrid, a signal output circuit coupled to the plate, means for feedingback to the screen grid from the output circuit a signal potential indegenerative phase with the signal potential applied to the signal grid,said feedback means comprising a potential divider including the screengrid to cathode resistance of said tube, and means for controlling thebias of said signal grid in a sense to decrease the tube gain thereby toincrease the magnitude of said resistance whereby the value of saiddegenerative potential is increased.

3. In a.signal amplifier of the type comprising a tube having at least acathode, signal grid, screen grid and plate, a signal input circuitcoupled to the signal grid, a signal output circuit coupled to theplate, means for feeding back to the screen grid from the output circuita signal potential in degenerative phase with the signal potentialapplied to the signal grid, said feedback means comprising a potentialdivider including the screen grid to cathoderesistance of said tube, andmeans responsive to signal amplitude increase at the signal inputcircuit for increasing the signal grid bias of said tube whereby saidresistance magnitude is increased and thereby increasing the magnitudeof said degenerative potential.

4. In a signal amplifier of the type comprising a tube having at least acathode, signal grid, screen grid and plate, a signal input circuitcoupled to the signal grid, a signal output circuit coupled to theplate, and means for feeding back to the screen grid from the outputcircuit a signal potential in degenerative phase with the signalpotential applied to the signal grid, said feedback means including aresistive impedance whose magnitude is substantially equal to the screengrid to cathode resistance and means for varying the gain of said tubein such a manner as to vary the magnitude of said resistancesufficiently to adjust said degenerative potential value.

5. In a radio receiver provided with a detector having a modulatedcarrier input circuit and a modulation voltage output circuit, amodulation voltage amplifier tube of the screen grid type, meanscoupling the detector output circuit to the modulation amplifier inputelectrodes, an output circuit coupled to the output electrodes of themodulation amplifier tube, means feeding to the screen grid of thelatter from the last named output circuit a modulation voltage indegenerative phase with the modulation voltage at the said inputelectrodes, said feeding means comprising a voltage divider whichincludes the screen grid to cathode resistance of said amplifier tube,and-means for simultaneously controlling the gain of said modulationamplifier tube in such a manner as to adjust the magnitude of saidresistance and thereby control the value of the degenerative voltage.

6. In a radio receiver provided with a detector having a modulatedcarrier input circuit and-a modulation voltage output circuit, amodulation voltage amplifier tube of the screen grid type, meanscoupling the detector output circuit to the modulation amplifier inputelectrodes, an output circuit coupled to the output electrodes of themodulation amplifier tube, means feeding to the screen grid of thelatter from the last named output circuit a modulation voltage indegenerative phase with the modulation voltage at the said inputelectrodes, and said feeding means comprising a voltage deviderincluding a resistor whose value is at least as great as the screen gridto cathode resistance of the modulation amplifier tube and means forcontrolling the amplifier tube gain in such a manner as to adjust thevalue of said resistance over a substantial range of values.

7. In a wave repeater circuit, a tube provided with at least a cathode,an input electrode, an output electrode and an auxiliary cold electrodein the electron stream to said output electrode, a wave input circuitcoupled to said input electrode and cathode, a wave output circuitcoupled to the output electrode and cathode, means for applying to saidauxiliary electrode wave potentials in desired degenerative phaserelation to the wave potential applied to the input electrode, saidapplying means comprising a potential divider which includes theresistance existing in said tube between the cathode and said auxiliaryelectrode, and means for controlling the input electrode bias of saidtube thereby simultaneously to control the magnitude of said resistanceand the magnitude of said auxiliary electrode wave potential in inverserelation.

, LO'I'HAR BRUCK.

